We have five primary characters. Of those five, three will be "new" to the audience.
This means that the film will need to introduce us to three new heroes and the villain(s).
At the moment, the DC universe does not have many villains/villain groups capable of matching the Justice League. We could go the multi-villain route, but that would require setting up around 5 villains to take on the five heroes. That's eight major characters introduced in one film. Our other alternative would be to introduce a single villain capable of delivering enough firepower to require five heroes. There aren't many villain groups like that in the DC universe, with Darkseid being the best known.
So based on the practical requirements of the film, if you don't want an epic monstrosity, you'd need to go the Darkseid route. Darkseid has faceless minions strong enough to provide fight, and enough power to to require the assembly of a Justice League.
If I was going to write a comic book, I would choose the opposite. Since comic books love to run many books, getting more villains is a bonus. I would have a group of villains ally together, learning to work together, just as the Justice League must learn to work together. When the showdown comes, both teams should have their issues worked out, meaning that the fight should feel like anybody's guess (even if we do know that the villains will lose). The villains will lose not because they aren't capable of winning, but when the stresses build enough, their seemingly settled problems flare up, and their base distrust of each other proves their undoing.